Fragile-X syndrome includes mental and physical defects, and is the most common form of inherited mental retardation (reviewed in: 1, 2-5). Defects have been observed in dendritic spines (6), and there are defects in the number and function of synapses (7). The disease is associated with loss-of-function mutations in the FMRI gene, usually by trinucleotide repeat expansion (8-12). FMR1 encodes an RNA binding protein (FMRP), which is widely expressed and has been proposed to act in the regulation of mRNA trafficking from the nucleus to specific cytoplasmic compartments and/or to have a role in the regulation of translation (13-27). There are two similar genes in humans (FXR1 and FXR2), and their products may act somewhat redundantly with FMRP (28-30). Some other proteins have been found in a ribonucleoprotein (RNP) complex with FMRP, and there are likely to be more (22, 31-33). A mouse model has been developed (by gene disruption), but this has produced some contradictory results (34-43). A single FMR1 homolog has been found in Drosophila, and ectopic expression of this protein in the fly causes developmental defects (44). While progress has been made in identifying protein partners of FMRP and its mRNA targets, this work has been difficult and there are likely to be many more genes that interact functionally with FMR1. We propose to discover and characterize novel genes that interact with FMR1 towards the long-term objective of improved therapeutic intervention. We have characterized true loss-of-function mutations in the Drosophila gene (Fmr1) and have studied the effects of the ectopic expression of the mouse FMR1 homolog in the fly. We have developed an F1 genetic screen based on the modification of the Drosophila Fmrl mis-expression phenotype in the compound eye, and pilot runs have validated our approach. We now propose two specific aims: 1) To complete the genetic screens, and map and characterize the resulting mutations at the genetic and phenotypic levels. 2) To begin the molecular isolation of the genes discovered and to correlate these to homologs in the human genome.